/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.common.primitives.Booleans;
import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;

import java.util.Comparator;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A utility for performing a chained comparison statement. For example:
 *
 * <pre>{@code
 * public int compareTo(Foo that) {
 *   return ComparisonChain.start()
 *       .compare(this.aString, that.aString)
 *       .compare(this.anInt, that.anInt)
 *       .compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())
 *       .result();
 * }
 * }</pre>
 *
 * <p>The value of this expression will have the same sign as the <i>first nonzero</i> comparison
 * result in the chain, or will be zero if every comparison result was zero.
 *
 * <p><b>Note:</b> {@code ComparisonChain} instances are <b>immutable</b>. For this utility to work
 * correctly, calls must be chained as illustrated above.
 *
 * <p>Performance note: Even though the {@code ComparisonChain} caller always invokes its {@code
 * compare} methods unconditionally, the {@code ComparisonChain} implementation stops calling its
 * inputs' {@link Comparable#compareTo compareTo} and {@link Comparator#compare compare} methods as
 * soon as one of them returns a nonzero result. This optimization is typically important only in
 * the presence of expensive {@code compareTo} and {@code compare} implementations.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/CommonObjectUtilitiesExplained#comparecompareto"> {@code
 * ComparisonChain}</a>.
 *
 * @author Mark Davis
 * @author Kevin Bourrillion
 * @since 2.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class ComparisonChain
{
    private ComparisonChain()
    {
    }

    /**
     * Begins a new chained comparison statement. See example in the class documentation.
     */
    public static ComparisonChain start()
    {
        return ACTIVE;
    }

    private static final ComparisonChain ACTIVE =
            new ComparisonChain()
            {
                @SuppressWarnings("unchecked") // unsafe; see discussion on supertype
                @Override
                public ComparisonChain compare(Comparable<?> left, Comparable<?> right)
                {
                    return classify(((Comparable<Object>) left).compareTo(right));
                }

                @Override
                public <T extends @Nullable Object> ComparisonChain compare(
                        @ParametricNullness T left, @ParametricNullness T right, Comparator<T> comparator)
                {
                    return classify(comparator.compare(left, right));
                }

                @Override
                public ComparisonChain compare(int left, int right)
                {
                    return classify(Ints.compare(left, right));
                }

                @Override
                public ComparisonChain compare(long left, long right)
                {
                    return classify(Longs.compare(left, right));
                }

                @Override
                public ComparisonChain compare(float left, float right)
                {
                    return classify(Float.compare(left, right));
                }

                @Override
                public ComparisonChain compare(double left, double right)
                {
                    return classify(Double.compare(left, right));
                }

                @Override
                public ComparisonChain compareTrueFirst(boolean left, boolean right)
                {
                    return classify(Booleans.compare(right, left)); // reversed
                }

                @Override
                public ComparisonChain compareFalseFirst(boolean left, boolean right)
                {
                    return classify(Booleans.compare(left, right));
                }

                ComparisonChain classify(int result)
                {
                    return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
                }

                @Override
                public int result()
                {
                    return 0;
                }
            };

    private static final ComparisonChain LESS = new InactiveComparisonChain(-1);

    private static final ComparisonChain GREATER = new InactiveComparisonChain(1);

    private static final class InactiveComparisonChain extends ComparisonChain
    {
        final int result;

        InactiveComparisonChain(int result)
        {
            this.result = result;
        }

        @Override
        public ComparisonChain compare(Comparable<?> left, Comparable<?> right)
        {
            return this;
        }

        @Override
        public <T extends @Nullable Object> ComparisonChain compare(
                @ParametricNullness T left, @ParametricNullness T right, Comparator<T> comparator)
        {
            return this;
        }

        @Override
        public ComparisonChain compare(int left, int right)
        {
            return this;
        }

        @Override
        public ComparisonChain compare(long left, long right)
        {
            return this;
        }

        @Override
        public ComparisonChain compare(float left, float right)
        {
            return this;
        }

        @Override
        public ComparisonChain compare(double left, double right)
        {
            return this;
        }

        @Override
        public ComparisonChain compareTrueFirst(boolean left, boolean right)
        {
            return this;
        }

        @Override
        public ComparisonChain compareFalseFirst(boolean left, boolean right)
        {
            return this;
        }

        @Override
        public int result()
        {
            return result;
        }
    }

    /**
     * Compares two comparable objects as specified by {@link Comparable#compareTo}, <i>if</i> the
     * result of this comparison chain has not already been determined.
     *
     * <p>This method is declared to accept any 2 {@code Comparable} objects, even if they are not <a
     * href="https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html">mutually
     * comparable</a>. If you pass objects that are not mutually comparable, this method may throw an
     * exception. (The reason for this decision is lost to time, but the reason <i>might</i> be that
     * we wanted to support legacy classes that implement the raw type {@code Comparable} (instead of
     * implementing {@code Comparable<Foo>}) without producing warnings. If so, we would prefer today
     * to produce warnings in that case, and we may change this method to do so in the future. Support
     * for raw {@code Comparable} types in Guava in general is tracked as <a
     * href="https://github.com/google/guava/issues/989">#989</a>.)
     *
     * @throws ClassCastException if the parameters are not mutually comparable
     */
    public abstract ComparisonChain compare(Comparable<?> left, Comparable<?> right);

    /**
     * Compares two objects using a comparator, <i>if</i> the result of this comparison chain has not
     * already been determined.
     */
    public abstract <T extends @Nullable Object> ComparisonChain compare(
            @ParametricNullness T left, @ParametricNullness T right, Comparator<T> comparator);

    /**
     * Compares two {@code int} values as specified by {@link Ints#compare}, <i>if</i> the result of
     * this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(int left, int right);

    /**
     * Compares two {@code long} values as specified by {@link Longs#compare}, <i>if</i> the result of
     * this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(long left, long right);

    /**
     * Compares two {@code float} values as specified by {@link Float#compare}, <i>if</i> the result
     * of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(float left, float right);

    /**
     * Compares two {@code double} values as specified by {@link Double#compare}, <i>if</i> the result
     * of this comparison chain has not already been determined.
     */
    public abstract ComparisonChain compare(double left, double right);

    /**
     * Discouraged synonym for {@link #compareFalseFirst}.
     *
     * @since 19.0
     * @deprecated Use {@link #compareFalseFirst}; or, if the parameters passed are being either
     * negated or reversed, undo the negation or reversal and use {@link #compareTrueFirst}.
     */
    @Deprecated
    public final ComparisonChain compare(Boolean left, Boolean right)
    {
        return compareFalseFirst(left, right);
    }

    /**
     * Compares two {@code boolean} values, considering {@code true} to be less than {@code false},
     * <i>if</i> the result of this comparison chain has not already been determined.
     *
     * @since 12.0
     */
    public abstract ComparisonChain compareTrueFirst(boolean left, boolean right);

    /**
     * Compares two {@code boolean} values, considering {@code false} to be less than {@code true},
     * <i>if</i> the result of this comparison chain has not already been determined.
     *
     * @since 12.0 (present as {@code compare} since 2.0)
     */
    public abstract ComparisonChain compareFalseFirst(boolean left, boolean right);

    /**
     * Ends this comparison chain and returns its result: a value having the same sign as the first
     * nonzero comparison result in the chain, or zero if every result was zero.
     */
    public abstract int result();
}
